We hypothesize that macrophages are able to influence the function of natural killer (NK) cells via a novel cytokine known as IL-15. Macrophages and natural killer cells comprise an important arm of the innate immune system and do not require prior sensitization or recognition of specific antigen for their function. Macrophages produce IL-12 and TNF- alpha in response to many infectious stimuli and these cytokines in turn drive the secretion of IFN-gamma by NK cells. IFN-gamma is a potent activator of macrophages and is critical for the early control of many pathogenic organisms. IL-2 induces a maximal production of IFN-gamma by NK cells in many of these systems, but IL-2 is produced solely by activated T cells and is therefore unlikely to be available to NK cells in vivo. Recently, Grabstein et al have cloned a novel cytokine, IL-15, which has no sequence homology to IL-2 yet binds to components of the IL-2 receptor. Importantly, IL-15 is -abundantly expressed in activated macrophages. Preliminary data will be presented which suggest that IL-15, in conjunction with IL-12, plays an important role in human NK cell homeostasis and activation, including the production of IFN-gamma. Thus, we hypothesize that the functions of NK cells in vivo are primarily controlled by macrophages and that IL-15 and IL-12 are important in the regulation of NK cell activity. Human macrophages will be cultured with various stimulatory factors and the regulation of IL-15 production will be characterized using ELISA techniques and northern blot analysis. To determine the contribution of IL-15 to NK cell production of IFN-gamma, co-cultures of macrophages and NK cells will be stimulated with macrophage activating factors and NK cells will be monitored for secretion of IFN- gamma in the presence or absence of IL-15 and IL-12 neutralizing antibodies. Activation of the IL-2 receptor appears to be important in the maintenance of NK cell viability. Therefore, we will investigate the ability of IL-15 to inhibit programmed cell death in NK cells using flow cytometry to analyze NK cellular DNA content and Bcl-2 levels. Finally, the role of IL-15 in a unique cytokine signalling pathway will be investigated using the electrophoretic mobility shift assay to analyze IL- 15-induced DNA binding proteins. Understanding the regulation of IL-15 production, its functional effects on human NK cells and the molecular mechanisms it utilizes to participate in innate immunity will have direct implications not only for our understanding of the human immune response, but also for biotherapeutic strategies directed against pathogenic organisms and malignant cells.